The Renaissance may be vivid in the mind's eye - in images of human figures sculpted in the round, or in scenes painted with a profound and moving realism. But as a concept it is a slippery customer.

The word is French for 'rebirth'. Historians first use it (from about 1840) for the period from the 14th to the 16th century, implying a rediscovery of rational civilization (exemplified by Greece and Rome) after the medieval centuries - seen as superstitious and artistically primitive. The term 'Middle Ages', also coined by historians, makes the same point in a different way - defining the medieval period merely as the gap between classical and modern civilization.

The first problem with this scenario is that the Middle Ages have a vivid cultural identity of their own, different from the classical pattern but not necessarily inferior. And the later medieval centuries, in particular the 12th and 13th, are unmistakably civilized.

The second difficulty is that it is impossible to establish clear dividing lines between medieval and Renaissance. In art (particularly sculpture) stylistic hints of the coming Renaissance can be seen well before 1300. But there is one field in which a new start is consciously made in the 14th century. This is the revival of the study of classical literature.

Petrarch the Laureate: 1341

On the Capitol in Rome, in 1341, a ceremony deliberately echoes the ancient Roman empire. The king of Naples, ruling in Rome on behalf of the pope in Avignon, places a laurel wreath on the brow of Petrarch - honouring him just as Augustus might have honoured Virgil.

The event deliberately symbolizes a renewed interest in classical culture, a movement in which Petrarch is a leading figure. But the new poet laureate adds a contemporary touch. He immediately goes to the tomb of St Peter and places on it his wreath.

This blending of the old and the new Rome, using the classical tradition in the service of Christianity, becomes a characteristic of Renaissance painting and sculpture. Christian saints are sculpted with the freshness of classical boys (Donatello's Saint George, for example), and painters place the gospel scenes in ancient Roman settings.

The roots of these artistic developments are too complex to be explained by a simple interest in classical culture. Only in the world of learning is the link between the Renaissance and the ancient world unmistakably clear. Only among Petrarch and his followers in the 14th and 15th century is the rebirth of the past (rinascimento in Italian) a conscious aim.

Petrarch, Boccaccio and humanism: 14th - 15th c.

In Florence, in April 1350, Petrarch makes his first influential convert to the cause of classical studies. He is visited by an admirer, Boccaccio, nine years younger than himself, who has written a biography of Petrarch but has not previously met him.

The encounter changes Boccaccio's life. He is in the middle of writing the work for which he is now famous, the Decameron. After completing it, probably in the following year, he abandons Italian literature - writing henceforth only in Latin and devoting himself to tracking down original manuscripts of classical texts.

Boccaccio is just one of the many followers of Petrarch who visit ancient monastery libraries in search of forgotten Latin manuscripts. They travel to Constantinople to bring back trunkloads of Greek parchments. They clamber among ancient ruins to note the inscriptions.

They copy out their findings and present their manuscripts to friends (soon the invention of printing will greatly speed up the spread of these texts). They form academies (echoing Plato's academy) in which they read learned papers on classical themes. They attempt performances of music and drama in what they believe to be the classical style. The members of one academy in Rome are even arrested for indulging in pagan classical rites.

Scholars of this kind become known as humanists, implying an admiration for the finest achievements of the human race. Human excellence and virtue is now seen as valuable in itself, in this present world of ours, rather than as a necessary qualification for entry to a world beyond.

An emphasis on the next world has characterized medieval teaching, broadly described as scholasticism. Humanism, in contrast to scholasticism, represents the cast of mind of the Renaissance. Beginning as a movement in Italy in the 14th century, it finds some of its greatest adherents in northern Europe as late as the 16th century - in influential figures such as Erasmus and Thomas More.

Roman and italic: 15th century

Italian scholars of the 14th and 15th century, followers of Petrarch in their reverence for classical culture, search through libraries for ancient texts. Copying out their discoveries, they aspire also to an authentic script. They find their models in beautifully written manuscripts which they take to be Roman but which are in fact Carolingian.

The error is a fortunate one. The script devised for Charlemagne's monastic workshops in the 8th century is a model of clarity and elegance. It is adapted for practical use, in slightly different ways, by two Florentine friends - Poggio Bracciolini and Niccolò Niccoli.

Bracciolini, employed as secretary at the papal court in Rome from 1403, uses the ancient script for important documents. To the rounded lower-case letters of the the Carolingian script he adds straight-edged capital letters which he copies from Roman monuments.

By contrast his friend Niccoli adapts the Carolingian script to the faster requirements of everyday writing. To this end he finds it more convenient to slope the letters a little (the result of holding the pen at a more comfortable angle), and to allow some of them to join up. Joining up is not in itself new. In several forms of medieval hand-writing the letters flow together to become what is known as a 'cursive' hand.

Printers in Venice later in the century, attempting to reflect the classical spirit of humanism, turn to the scripts of Bracciolini and Niccoli. The rounded but upright style of Bracciolini is first used by the French printer Nicolas Jenson shortly after his arrival in the city in 1470. This type face is given the name roman, reflecting its ancient origins.

In 1501 another great Venetian printer, Aldus Manutius, needs a contrasting and smaller type for a 'pocket edition' of Virgil. He turns to the script of Niccoli, in everyday use by fashionable Italians, and calls it accordingly italic. Roman and italic eventually become a standard part of every printer's repertoire.